1. Field of the Invention
This invention relates to a method of producing dialkylcarbonate using both an alcoholic compound and a cyclocarbonate as raw material in the presence of an ion-exchanged zeolite which is ion-exchanged with an alkali metal ion and/or alkaline earth metal ion.
2. Description of the Related Art
As a method of producing dialkylcarbonate through reaction of alcohol and cyclocarbonate, there has been conventionally known a method of reacting methanol with ethylene carbonate in the presence of a homogeneous or heterogeneous catalyst to produce dimethylcarbonate and ethylene glycol.
In the method utilizing the homogeneous catalyst, an amine such as triethylamine, an alkali metal such as sodium, an alkali metal compound such as sodium chloroacetate or sodium methylate, or a thallium compound has been used as the catalyst for the reaction.
In the method utilizing the heterogeneous catalyst, ion exchange resins have been conventionally proposed as catalysts. For example, Japanese Laid-open Patent Application No. 64-31737 proposes the heterogeneous catalysis using, as a catalyst, ion exchange resins having various kinds of functional groups, amorphous silica into which alkali and alkaline earth metal silicate are impregnated, or ammonium-ion-exchanged zeolite having a Y-structure (hereinafter referred to as Y-zeolite).
In the conventional reaction methods using the catalysts as described above, the reaction method using the heterogeneous catalysis is more preferably used for industrial application than the reaction method using the homogeneous catalysis because the reaction product mixture and catalyst can be more easily separated from each other in the former than in the latter.
However, when the ion exchange resins or the silica impregnated with alkali and alkaline earth metal silicate as described above are used as the catalyst, catalytic active sites irregularly exist, and thus they are inhomogeneously distributed. Therefore, it is considered that catalytic activity cannot be improved using these catalysts.
On the other hand, in using a zeolite catalyst it is expected that homogeneous catalytic active sites can be formed because cations can be contained in polyanionic framework cavities of the three-dimensional framework structure of aluminosilicic acid constituting crystalline aluminosilicate. Generally, positive ions exist to balance the electrostatic charge in the polyanionic framework structure of the zeolite. Further, the zeolite catalyst has excellent heat-resistance and thus it is usable for a reaction at a high temperature if occasion demands, so that it is considered to be favorable as a catalyst.
However, the catalytic activity of the above-described ammonium-ion exchanged Y-zeolite catalyst is considered to be low because the component is volatile and thus the active sites are unstable. Actually, in a comparison example as described later, the appearance of catalytic activity of the zeolite catalyst for a conversion reaction was not observed at a reaction temperature of 50.degree. C.
An ion exchange resin catalyst has inferior organic solvent resistance, especially upon heating. The ion exchange resin catalyst gradually deteriorates, and finally loses catalytic activity due to elution of the functional groups and loss of catalytically active sites over a long period of service.
When the catalytic activity is low, the reaction temperature must be increased in order to raise the reaction rate and improve conversion efficiency to make up for lack of catalytic activity. However, the ion exchange resin has low heat-stability as above described, and thus the reaction temperature is limited to about 100.degree. C. at maximum. Particularly for the ammonium-ion exchanged zeolite catalyst, the reaction temperature cannot be increased while stably keeping the catalytic activity thereof because the ammonium component is volatile. Therefore, process control and operation are complicated for an industrial use of the ion exchange resins and the ammonium-ion exchanged zeolite.